Method for the acetylation of wood

ABSTRACT

The present invention provides a method for the acetylation of wood comprising treating the wood with an acetylation medium under wood acetylation reaction conditions and drying the acetylated wood, wherein the drying comprises at least two steps, wherein the wood is first dried with a first drying medium and then with a second drying medium.

The invention relates to the field of wood acetylation. Particularly,the present invention provides a method for the acetylation of woodincluding a step of drying the acetylated wood.

BACKGROUND OF THE INVENTION

Acetylation of wood is used to improve the durability of wood and ofwood elements used in the preparation various products such as fibreboards. Acetylation is typically performed in a batch or continuous typeof reactor. After the acetylation process is finished, the acetylatedwood is typically dried to remove the chemicals.

When wood is acetylated in industrial amounts, it is a general desire toacetylate in a relatively quick and economical way. This especiallyholds for continuous processes that often use a particular residencetime of the wood in the reactor. It is therefore desired that also thedrying step following the acetylation step is performed in a quick andefficient way.

It is an object of the present invention to provide a method for theacetylation of wood that allows fast and efficient removal of theresidual chemicals from the acetylated wood.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention presents, a method for the acetylation of wood comprisingtreating the wood with an acetylation medium under wood acetylationreaction conditions and drying the acetylated wood, wherein the dryingcomprises at least two steps, wherein the wood is first dried with afirst drying medium and then with a second drying medium.

DETAILED DESCRIPTION OF THE INVENTION

The invention, in a broad sense, is based on the judicious insight thatthe drying of acetylated wood following an acetylation process can becarried out more efficiently in several steps, using different dryingmedia. Drying in the present invention refers to a post-reaction step,that is, after the acetylation reaction, or after all the acetylationreaction steps in case several reaction steps are carried out. Duringthis post-reaction drying residual chemicals are removed from theacetylated wood (particularly, the acetylation medium and reactionby-products). In some embodiments, drying aims to remove the residualamounts of acetic acid and acetic anhydride after the acetylationstep(s). Drying as used throughout this application is hence distinctlydifferent from removing water from non-acetylated wood, as often done ina pre-acetylation step.

According to the present invention, the wood is dried in at least twosteps. It is first dried with a first drying medium and then with asecond drying medium, optionally wherein the first drying medium has ahigher heat capacity than the second drying medium and/or the firstdrying medium comprises a higher content (mol %) of acetylation mediumthan the second drying medium. While some conventional techniques use ahot inert gas for drying, it may take considerable time to lower theamount of impurities to an acceptable level. A longer drying time isespecially applicable to high pressure acetylation processes becauseafter the pressure release following the acetylation step, the wood iscooled down due to flashing and needs to be reheated. If the drying isdone using an inert gas such as nitrogen, due to its low heat capacityit takes more time before the wood is dried.

This problem is solved by first drying with a first, preferably highheat capacity drying medium and then drying with a second drying medium.The first drying medium preferably has a higher heat capacity and servestwo goals. Firstly, it efficiently heats up the wood and, secondly, itis able to remove the chemicals from the wood to a certain level. Whilethat level may still be too high compared to industrial requirements,using another drying medium, preferably an inert gas, in a second stepallows to lower the content of the chemicals to a sufficiently lowlevel, which meets industry standards (<0.5 wt. % for residual aceticacid). A combination of both steps, therefore, achieves a quick andefficient drying of acetylated wood to a low residual acetic acidcontent. The use a an acetylation medium in the first step also enablesefficient integration of drying purge gases in the acetylation processwhilst also minimizing the use, and subsequent purification, of inertgases. The use of an inert gas in the final stage enables low residualhydrocarbon levels. Thus the judicious selection of drying medium,optionally recycle and purge gas flows and residence time in each dryingstage enables an improved drying efficiency.

The wood suitable for the method according to the invention can beeither solid wood or wood elements. Solid wood would typically have awidth and a thickness of from 1 to 30 cm, and a length of from 1 to 6 m.Preferably, the wood has a width and a thickness of from 2 to 10 cm anda length of 1.5-4 m. However, other dimensions can also be used.

In a preferred embodiment, the wood is in the form of wood elements.Preferably, the wood elements have a size of a height and width from 0.1cm to 3 cm, and a length of 1 to 7 cm. In some embodiments it may bepreferred to have the wood elements with a height and width from 1 to2.5 cm, and a length of 2-5 cm, for example approximately 2.5 cm×1.5cm×1.5 cm in size. The morphology of the wood elements may be, but isnot limited to, wood chips, wood fibres, wood flour, wood strands, woodshavings, etc. In a preferred embodiment, the wood elements are woodchips. The wood or wood elements preferably belong to non-durable woodspecies such as soft woods, for example, coniferous trees, spruce, pineor fir or to non-durable hardwoods, typically spruce, pine or fir beech,birch, eucalyptus, poplar or alder.

Preferably, the wood has a moisture content of at most 6 wt. % dry base,before the acetylation. After chipping the trees, the green wood has atypical natural moisture content (MC) of >100 wt. % based on the dryweight. The moisture content of the wood is preferably lowered to below6 wt. % dry weight, before the wood is treated in the acetylationprocess. At higher moisture contents too much of the acetylation medium(typically containing acetic anhydride) will be consumed in a reactionwith water in the wood instead of with the hydroxyl groups in the wood,which makes the acetylation process less efficient. Another disadvantageof a too high MC is that the strong exothermic reaction of aceticanhydride with water in the wood can lead to local overheating in thewood and discoloration of the wood may occur. More preferably, themoisture content is less than 4 wt. %, ideally less than 3 wt. %.Lowering of the moisture content can be achieved by conventionalcontinuous or batch drying techniques.

According to the method of the present invention, the wood is acetylatedto obtain acetylated wood. Acetylation is achieved by treating the woodwith an acetylation medium under wood acetylation reaction conditions.Under “acetylation medium” it is understood a compound or a mixture ofcompounds that are able to acetylate the hydroxyl groups of cellulose inthe wood. For example, acetic acid or acetic anhydride are suitableacetylation media. Preferably, the acetylation medium comprises aceticacid, acetic anhydride or a mixture thereof. More preferably, theacetylation medium comprises a mixture of acetic acid and aceticanhydride. Particularly, such mixture preferably comprises at least 50wt. %, more preferably at least 85 wt. % of acetic anhydride.

Under “wood acetylation reaction conditions” the conditions areunderstood under which the wood undergoes acetylation. A skilled personis aware of specific temperature and pressure conditions to achieveacetylation of wood. Usually acetylation is performed under an elevatedtemperature, e.g. at a temperature 70-190° C. An example of anacetylation process is the one described in EP 680810. Preferably, theacetylation conditions comprise heating at a temperature in the range160-190° C.

In a particularly preferred embodiment, the acetylation takes place atan increased pressure, preferably at least 1 barg. More preferably, thepressure in the acetylation reactor is in the range 1.5-5 barg, yet morepreferably 2-3.5 barg. When the acetylation takes place under anincreased pressure, the pressure is ideally released before entering thefinishing step, i.e. prior to drying. Since the wood still containsliquid from the acetylation step after the acetylation, the release ofthe pressure will result in a rapid cooling of the wood due to flashing.In the method according to the present invention, the two-step dryingprovides for a rapid and efficient re-heating of the cooled wood to thetemperature of drying and removal of remaining chemicals from the wood.

The unreacted acetylation medium from the acetylation step is preferablyrecycled and re-used in the process. This can be realised by condensingthe vapour at the exit part of the reactor and separating bydistillation of acetic anhydride. Separated acetic anhydride can then bere-used in the process and separated acetic acid can be stored or usedin another process, or converted to acetic anhydride by known methods.

In case the acetylation is conducted at an elevated pressure, thepressure is substantially released at the end of the acetylationreaction. Preferably the pressure drops to about −0.4 barg to 0.3 barg.

In one embodiment of the present process the acetylated wood at the endof acetylation contains at least 55 wt. % liquid on dry basis of theacetylated wood weight. When an elevated pressures is used, theabove-mentioned liquid content refers to the wood beforedepressurization.

In one embodiment, depressurisation is achieved by discharging theacetylated material from the reactor using a an arrangement of one ormore rotary valves and/or sluice gates.

In a further step, the acetylated wood is dried. Drying means theremoval of the acetylation medium and reaction products from theacetylated wood. During drying acetic acid and unreacted aceticanhydride are removed, preferably to a level of below 0.5 wt. %.

An important characteristic of the present invention is that the dryingcomprises at least two steps wherein different drying media are used.

In a first step, the wood is dried with a first drying medium, whichpreferably comprises an acetylation medium vapour and more preferablycomprises superheated acetylation medium vapour. Superheated mediummeans heated to a temperature higher than its boiling point, withoutboiling. By its nature, the superheated acetylation medium is suitablefor drying the acetylated wood containing liquid acetylation medium andother acetylation residues. The first drying medium has preferably ahigher heat capacity than the heat capacity of the second drying medium,used in a following step. The first drying medium preferably comprises ahigher content (mol %) of acetylation medium than the second dryingmedium.

The acetylation medium vapour preferably comprises acetic acid, aceticanhydride or a mixture thereof. Suitably, the acetylation medium vapourcan comprise the unreacted acetylation medium from the acetylation step,which is preferably recycled an re-used in the process.

The first drying medium preferably comprises at least 90 wt. % of theacetylation medium (preferably, a mixture of acetic acid and aceticanhydride) and a maximum of 10 wt. % of an inert gas such as nitrogen orthe like. Preferably, the acetylation medium is a mixture of acetic acidand acetic anhydride. An example of a preferred composition of the firstdrying medium is 20-50 wt. % acetic acid, 50-80 wt. % acetic anhydrideand 0-3 wt. % of nitrogen.

Preferably, the residence time in the first drying step is 5-25 min. Thegas to solid flow rate in first drying step is preferably 1-6 massratio.

In a second drying step, the wood is dried with a second drying medium,which is preferably an inert gas. Inert gas means that it is inert tothe wood or the acetylation reaction. Preferably, the inert gas isnitrogen, carbon dioxide or flue gas. More preferably, the inert gas isnitrogen.

In both steps, conventional drying techniques can be used, for exampleblowing with a (hot) gas over the wood. Preferably, the acetylated woodis dried in either or both of the drying steps at a temperature in therange 150-190° C., more preferably in the range 160-180° C. The dryingin the first and/or the second step is preferably carried out at apressure lower than the acetylation reaction pressure. Preferably,ambient pressure is used during drying, more preferably the pressurefrom −0.4 barg to 0.3 barg is used.

The residence time in the second drying step is preferably 15-40 min.The gas to solid flow rate in first drying step is preferably 0.7-5 massratio.

Preferably, the acetylated wood before discharging or depressurizationcontains at least 55 wt. % liquid on dry basis of the acetylated woodweight; after flashing—i.e. at beginning of the first drying step, theliquid content is typically reduced to at least 30 wt. %. During thefirst drying step this liquid content is typically reduced to 2-8 wt. %,at most 10 wt. %. During the second drying step this is further reducedto max 0.5 wt. % on dry basis.

The described two drying steps can suitably take place in vessels thatensure a minimum residence time for all wood elements and that providefor good, preferably uniform distribution, of the drying medium throughthe wood elements in the dryer. The drying medium may flow in acounter-current or cross-current manner relative to the wood elements.The drying medium used for each drying step is preferably recycled toachieve the required drying flowrate with minimum make-up gasrequirement. Each such recycle loop may be provided with (i) cooling tocondense and then separate degassed hydrocarbons from the recycleddrying medium and/or (ii) a vapor make-up or purge to maintain massbalance. The drying medium fed to each step is preferably heated priorto introduction into the drying vessel.

The drying vessel may be a batch or continuous, optionally a plug flow,vessel or silo. The drying vessel may alternatively be a screw devicewith a cross-flow of drying gas.

The acetylation reaction and the drying steps can be each performed in acontinuous manner or batchwise. Preferably, the acetylation reactionand/or each of drying steps are carried out as a continuous process,more preferably all the three mentioned steps are carried out in acontinuous manner. In an alternative embodiment, the acetylationreaction and/or any of the drying steps is carried out batchwise.Preferably, all the three mentioned steps are carried out batchwise.

One or more of the acetylation reaction and each of the drying steps maybe effected in the same or in separate vessels. In one embodiment, theacetylation reaction and each of the drying steps are performed in aseparate vessel, preferably with each step being continuous. In analternative embodiment, the acetylation reaction and each of the dryingsteps are performed in the vessel, preferably with each step beingbatchwise.

The dried acetylated wood can be cooled down to room temperature.Conventional techniques can be used for that such as air cooling orwater cooling.

An important advantage of the method according to the present inventionis that it allows the obtaining of acetylated wood with a highacetylation degree and low residual acid content in a quick andefficient way. Due to the above-described two-step drying procedure withdifferent drying media, the wood can efficiently and fast be (re-)heated and the residual chemicals be removed in a relatively shortperiod of time. The total drying time is preferably under 90 min, morepreferably in the range 20-65 min.

Preferably, the acetyl content of the acetylated wood is from 17% to26%, as measured by high-pressure liquid chromatography (HPLC),preferably at least 19%, more preferably 21%.

The residual acetic acid (RA) content is a measure of the residual,non-bound acetic acid or from hydrolysis of residuals of unreactedacetic anhydride contained in the wood. Acetic acid may also originatefrom the wood itself, therefore the RA measures both the original aceticacid and the acetic acid left from the acetylation reaction. Fordetermining the residual acid (RA) a well defined amount of 3-5 g ofsample material is shaken in demineralised water for 1 hr. After thisextraction step the sample is separated from the water fraction byfiltration. Subsequently this water fraction is titrated with a knownsodium hydroxide (NaOH) solution, using phenolphthalein as an indicator,from which the residual acid concentration of the sample can becalculated.

The method according to the present invention allows to achieve the RAof the acetylated wood below 1% by weight, preferably below 0.9%, morepreferably below 0.5% by weight. Most preferably, the acetylated woodcontains less than 0.2% of residual acid by weight. In a preferredembodiment, the RA content is from 0.05 to 0.9 wt. %. One of theadvantages of the low levels of residual acetic acid is that the wooddoes not have any odour of acetic acid.

Acetylated wood obtained according to the method of the presentinvention may be used in various applications. Acetylated wood elementsmay usefully be refined and converted to board, such as medium densityfibreboard, which will possess the superior dimensional stability,durability, stability to ultra-violet light and thermal conductivity,compared with boards derived from non-acetylated wood elements.

The following non-limiting example serves to illustrate the invention.In this example and throughout this specification, all percentages,parts and ratios are by weight unless indicated otherwise.

EXAMPLE

A mixture of wood chips with dimensions 2.5 cm×1.5 cm×1.5 cm consistingof 80% Sitka Spruce with 20% Loblolly Pine has been dried to a moisturecontent of 2.5%. This mixture is fed to an acetylation step where it issprayed with liquid acetic anhydride/acetic acid mixture and heated withsuperheated acetic anhydride/acetic acid mixture (where the liquid togas ratio is 1:2.5 wt:wt) to a temperature of 180° C. at a pressure of2.5 barg. The weight ratio of acetic anhydride to acetic acid in theacetylation medium mixture is 90:10. After a residence time of 30minutes at these conditions the pressure is released and the wood chipsare transferred to the drying step, which proceeds at 0 barg. The firstdrying step occurs with a gas flow to solid flow ratio of 3.2:1(mass:mass) with a gas phase composition of 33% acetic acid, 65% aceticanhydride and 2% nitrogen. The residence time for this step is 15minutes, where the chips are heated with a temperature of 170° C.Subsequently the chips are transferred to the second drying step, wherethe chips are heated with nitrogen at 170° C. for 25 additional minutes.This process resulted in acetylated wood chips with an acetyl content(AC) of 23.2% and a residual acid content of 0.3 wt. %.

1. A method for the acetylation of wood comprising treating the woodwith an acetylation medium under wood acetylation reaction conditionsand drying the acetylated wood, wherein the drying comprises at leasttwo steps, wherein the wood is first dried with a first drying mediumand then with a second drying medium.
 2. The method according to claim1, wherein the first drying medium has a higher heat capacity than thesecond drying medium.
 3. The method according to claim 1, wherein thefirst drying medium comprises an acetylation medium vapour.
 4. Themethod according to claim 1, wherein the second drying medium is aninert gas, preferably nitrogen.
 5. The method according to claim 1,wherein the acetylation reaction conditions comprise a pressure of atleast 1 barg.
 6. The method according to claim 1, wherein the drying inthe first and/or the second step is carried out at a pressure lower thanthe acetylation reaction pressure.
 7. The method according to claim 1,wherein the acetylation medium comprises acetic acid, acetic anhydrideor a mixture thereof.
 8. The method according to claim 4, wherein theacetylation medium comprises a mixture of acetic acid and aceticanhydride, preferably comprising at least 50 wt. %, more preferably atleast 85 wt. % of acetic anhydride.
 9. The method according to claim 1,wherein the acetylation conditions comprise heating to a temperature inthe range 160-190° C.
 10. The method according to claim 1, wherein thedrying temperature in either one or both of the drying steps is in therange 150-190° C.
 11. The method according to claim 1, wherein in thefirst drying step the residence time is 5-25 min and, optionally, thegas to solid flow rate is 1-6 mass ratio.
 12. The method according toclaim 1, wherein in the second drying step the residence time is 15-40min and, optionally, the gas to solid flow rate is 0.7-5 mass ratio. 13.The method according to claim 1, wherein the liquid content in theacetylated wood after acetylation and before drying is at least 55 wt. %on dry basis.
 14. The method according to claim 1, wherein the liquidcontent in the acetylated wood is reduced during the first drying stepto at least 2-8 wt. % on dry basis and in the second drying step to max0.5 wt. % on dry basis.